What's stopping Marvel from saying who exactly she is? Why would they hold that information back? Is she not Sharon possibly? Otherwise what other possible reason do they have for not just revealing her character? Is JJ Abrams working on this somehow?

Simple really, they haven't released their official press release regarding her casting yet. It's not really that they're being secretive or anything. I'm sure her character's identity will be in it when they do. I'd expect it within the next week or so.